Transmission Electron Microscopy.zip

Amyloids, classically linked to neurodegenerative diseases, also play critical roles in infection and immunity. Phenol-soluble modulins (PSMs) from Staphylococcus aureus are virulent amyloids that contribute to cytotoxicity, immune modulation, and biofilm stability. PSMα3 forms cross-α amyloid fibrils and shares sequence and structural features with LL-37, a human host-defense peptide that assembles into α-helical structures. Here, we uncover RNA as a potent, context-dependent modulator of their aggregation and activity. RNA consistently reduces LL-37’s cytotoxicity toward human cells without compromising its antibacterial function, suggesting a selective host-protective mechanism. In contrast, RNA preserves PSMα3’s cytotoxic and antimicrobial activity over time, likely by promoting liquid–liquid phase separation (LLPS) and stabilizing bioactive fibrillar polymorphs, enabling S. aureus to fine-tune its virulence strategies. At higher concentrations, RNA drives both peptides toward distinct aggregated states, amorphous for LL-37 and fibrillar for PSMα3, underlying their divergent functional outcomes. The amyloid inhibitor EGCG abolishes the bioactivity of both PSMα3 and LL-37, overriding RNA’s modulatory effects. Together, our findings establish RNA as a key modulator of both virulent amyloids and host-defense peptides, with broad implications for microbial immune evasion, innate immunity, and amyloid-associated diseases. Moreover, they highlight phase transitions as a tunable mechanism for regulating peptide bioactivity and a promising therapeutic target across infectious and neurodegenerative conditions.